Electroless plating solutions for cadmium and cadmium copper alloys

ABSTRACT

THIS INVENTION RELATES TO ELECTROLESS DEPOSITION OF CADMIUM AND ALLOYS OF CADMIUM AND COPPER. THE INVENTION IS BASED UPON TWO DISCOVERIES, THE FIRST BEING THAT UP TO ABOUT 25% BY WEIGHT CADMIUM CAN BE CODEPOSITED WITH COPPER BY THE SIMPLE SUBSTITUTION OF A CADMIUM SALT FOR A COPPER SALT IN AN ELECTROLESS COPPER DEPOSITING FORMULATION. THE SECOND DISCOVERY IS THAT IF THE ELECTROLESS COPPER SOLUTION ALSO CONTAINS A TRACE QUANTITY OF MERCURY, ANY PORTION OF THE COPPER SALT IN THE ELECTROLESS DEPOSITING SOLUTION CAN BE REPLACED BY THE CADMIUM SALT PROVIDED THAT A TRACE OF COPPER RETAINED TO INITIATE THE REACTION AND PROVIDED FURTHER, THAT THE SOLUTION ALSO CONTAINS A CHELETING AGENT FOR THE CADMIUM WHICH MAY BE THE SAME AS THE CHELATING AGENT FOR THE COPPER. IN ACCORDANCE WITH THE INVENTION, DEPOSITS CAN BE OBTAINED THAT ARE ESSENTIALLY PURE ELECTROLESS CADMIUM RICH CADMIUM-COPPER ALLOYS OR COPPER RICH COPPER-CADMIUM ALLOYS.

United States Patent 3,677,776 ELECTROLESS PLATING SOLUTIONS FOR CADMIUM AND CADMIUM COPPER ALLOYS Charles Raymond Shipley and Michael Gulla, Newton, and Oleh Borys Dutkewych, Medfield, Mass, assignors to Shipley Company, Inc., Newton, Mass. No Drawing. Filed Mar. 12, 1971, Ser. No. 123,840.

Int. Cl. C23c 3/02 U.S. Cl. 106-1 40 Claims ABSTRACT OF THE DISCLOSURE This invention relates to electroless deposition of cadmium and alloys of cadmium and copper. The invention is based upon two discoveries, the first being that up to about 25% by weight cadmium can be codeposited with copper by the simple substitution of a cadmium salt for a copper salt in an electroless copper depositing formulation. The second discovery is that if the electroless copper solution also contains a trace quantity of mercury, any portion of the copper salt in the electroless depositing solution can be replaced by the cadmium salt provided that a trace of copper is retained to initiate the reaction and provided further, that the solution also contains a chelating agent for the cadmium which may be the same as the chelating agent for the copper. In accordance with the invention, deposits can be obtained that are essentially pure electroless cadmium rich cadmium-copper alloys or copper rich copper-cadmium alloys.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to metal depositing compositions and more particularly, to electroless compositions capable of depositing electroless cadmium, electroless cadmium rich cadmium-copper alloys or electroless copper rich copper-cadmium alloys.

(2) Description of the prior art 7 Electroless metal deposition refers to the chemical plating of a metal, typically nickel or copper over a clean catalytically active surface by chemical reduction in the absence of an external electric current. Such processes and compositions useful therefor are known and are substantial commercial use. They are disclosed in a number of prior art patents. For example, copper formulations are disclosed in US. Pats. Nos. 2,938,805; 3,310,430 and 3,383,224. Nickel formulations are disclosed in US Pats. Nos. 2,532,283; 2,762,723; 2,935,425 and 2,999,777.. All of the foregoing patents are incorporated herein by reference.

Using copper as an example, known electroless solutions generally comprise at least four ingredients dissolved in a solvent, typically water. They are (l) a source of cupric ions, usually a copper salt such as copper sulphate, 2) a reducing agent such as formaldehyde, or preferably, a formaldehyde precursor such as paraformaldehyde, (3)

hydroxide, generally an alkali metal hydroxide and usually sodium hydroxide, sufficient to-provide the required alkaline solution in which said copper compositions are effective and (4) a complexing agent for copper sufiicient to prevent its precipitation in alkaline solution. A large number of suitable complexing agents for copper are known and described in the above cited patents and also in US. Pats. Nos. 2,874,072; 3,075,855; 3,075,856; 3,119,709 and 3,329,512 all in corporated herein by reference.

Compositions for depositing metals other than copper and nickel are also known in the art, such compositions 3,677,775 Patented July 18, 1972 STATEMENT OF THE INVENTION The invention disclosed herein provides new electroless depositing metals in that it provides formulations for depositing electroless cadmium, cadmium rich cadmiumcopper alloys, and copper rich copper-cadmium alloys. The invention is predicated upon two discoveries. The first discovery is that up to about 25% cadmium can be codeposited with copper by the simple substitution of a cadmium salt in part for a copper salt in an electroless copper depositing formulation. The second discovery is that if the electroless copper solution also contains a trace quantity of mercury ions, any portion of the copper salt in the electroless copper depositing formulation can be replaced by a cadmium salt provided a trace of the copper salt is contained in solution to initiate the reaction and provided further, that the complexing agent for the copper salt is also a complexing agent for the cadmium salt or a cadmium complexing agent is used in solution.

In accordance with the invention, an electroless depositing formulation will comprise (1) a source of cadmium ions, (2) a source of copper ions, (3) a complexing agent for the copper ions, (4) a complexing agent for the cadmium ions which may be the same as that used for the copper ions, (5) formaldehyde or a formaldehyde precursor such as paraformaldehyde as a reducing agent, (6) hydroxide sufficient to provide required solution pH and (7) a source of mercury.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Any soluble cadmium salt and copper salt having anions compatible with the electroless depositing solution may be used for purposes of this invention. 'For example, the halides, nitrate, acetate, sulphate and other organic and inorganic salts of copper and cadmium are generally suitable. Cadmium sulphate and copper sulphate are preferred.

The amount of cadmium salt relative to the amount of copper salt in solution is dependent to some extent upon the amount of cadmium and copper desired in the electrolessly deposited alloy. If a substantially pure cadmium deposit is desired, a trace of copper salt is required in the solution, typically at least 5 parts per million parts of solution and preferably about to 250 parts per million parts of solution. If a cadmium rich cadmium-copper alloy is desired, the amount of cadmium in solution should exceed the amount of copper. If a copper rich cadmiumcopper alloy is desired, the amount of copper in solution should exceed the amount of cadmium. Thus, the molar ratio of the two metals in solution may vary within broad limits dependent upon the actual alloy desired. In general, the preferred molar ratio of cadmium to copper varies from about 1:50 to 1000:1 and more preferably from about 1:1 to 500:1.

The combined amount of cadmium and copper in solution may vary within wide limits, typically from about 0.002 mole per liter to saturation, and preferably from about 0.02 to about 2.0 moles per liter of solution.

The formaldehyde or formaldehyde precursor is used to reduce both cadmium and copper contained in solution to metal. The amount of formaldehyde used is in accordance with prior art electroless copper solutions and typically varies from about 0.05 to 3.0 moles per liter of solution, and preferably from about 0.1 to .1 mole per liter of solution.

It is necessary that the solution contain complexing agents both for copper and cadmium in amounts corresponding to the amount of each metal contained in solution. Many complexing agents heretofore used in electroless copper solutions are complexing agents for cadmium also and consequently, may be used as the sole complexing agent for both metals. However, some complexing agents for copper, such as carboxylic acid type complexing agents, are not suitable complexing agents for cadmium in accordance with the invention and if used for copper, must be used in conjunction with an additional complexiug agent for cadmium. Suitable complexing agents for both copper and cadmium include the amine type complexing agents such as the sodium salts (mono-, di-, tri-, and tetra-) of ethylene diamine tetraacetic acid, nitrilotriacetic acid and its alkali metal salts, triethanolamine, modified ehtylene diamine tetraacetic acids such as N-hydroxyethylenediamine triacetate, hydroxyalkyl substituted dialkyleue triamines such as pentahydroxypropyl diethylenetriamine and the like. A preferred class of complexing agents are those described in US. Pat. No. 3,329,512 which discloses the use of hydroxyalkyl substituted tertiary amines. Other complexing agents for copper are known in the art and described in the above referenced patents. Other complexing agents for cadmium are identified in various handbooks.

The amount of complexing agent used is in accordance with prior art procedures for formulating electroless copper deposition solutions. In other words, the complexing agent is used in an amount sufiicient to at least maintain the metal ions in solution and preferably in an amount from about one to three times the moles of metal ions in solution.

The hydroxide is used to maintain the pH of the solution on the alkaline side and preferably at pH or greater. Alkali metal hydroxides are preferred such as sodium and potassium hydroxide.

Mercury is used in solution to catalyze the reaction. In this respect, where the amount of codeposited cadmium does not exceed 25% by weight of the deposit, then mercury is not required. However, where larger concentrations of cadium are required in the deposit or where substantially pure cadmium deposits are desired, it is necessary to have the mercury in solution as it appears to permit or catalyze the deposition of cadmium in larger concentrations. The reason for this is not understood, but is believed to be associated with the high hydrogen over-voltage of mercury.

Both simple and complex mercury compounds, at least partially soluble in the electroless plating solution, are suitable sources of mercury for purposes of the present invention. Typical examples of mercury compounds include mercuric acetate, mercurous acetate, mercuric ben zoate, mercurous bromide, mercuric bromide, mercuric carbonate, mercurous chlorate, mercurous chloride, mercuric chloride, mercuric iodate, mercuric nitrate, mercuric sulphate, ethylmercuric hydroxide, ethyl-mercuric mercaptide, mercuric phenylnrercaptide, methylmercuricchloride, and the like.

The amount of mercury compound required in the bath is not critical and it is suflicient to have mercury compound present in trace quantities only. Typically, the mercury compound may be in solution in an amount as low as 1 part per million parts of solution, but preferably varies from about 10 to 100 parts per million parts of solution. An amount up to saturation of the mercury compounds in solution is tolerable, but not necessary.

For reasons not fully understood, it has been found that the electroless plating baths of the invention provide better results in terms of reproducibility when containing certain polymeric materials. The polymers contemplated are those disclosed in US. Pat. No. 3,329,512. They are dispersible in alkaline solution, preferably as a colloid in solution, in the quantities required. Useful polymers include cellulose ethers, hydroxyethyl starch, polyvinyl alcohol, polyvinyl pyrrolidone and copolymers thereof. Other polymers are set forth in said patent. The molecular weight of the polymer does not appear to be critical, some of the polymers capable of use in liquid form and others in the form of higher molecular weight solids. In general, those polymers that are most useful have secondary or tertiary nitrogen atoms and/or hydroxyl groups and pos sess surfactant properties and an ability to increasethe viscosity of aqueous solutions.

The amount of polymer added to the electroless plating solution is not critical, from a few parts per million parts of solution up to several thousand parts per million parts of solution being satisfactory, the smaller quantities providing some benefits while the larger quantities provide greater benefits until the concentration reaches the point where the increased plating solution viscosity is too high. Preferably, the polymer is in solution in an amount of from about 10 parts per million parts of solution to about 600 parts per million parts of solution and most preferably, from about 20 parts per million parts of solution to about parts per million parts of solution.

The electroless solutions of this invention are used in accordance with art recobnized procedures for plating electroless metals. The bath may be used at widely varying temperatures, e.g., from about room temperature to about F. Room temperature is preferred though as the temperature of the solution is increased, it is customary to find an increase in the rate of plating. Temperature is not highly critical and within the usual operating ranges, bright deposits of electroless cadmium, cadmiumcopper alloy or copper-cadmium alloy are obtained.

In using the electroless solutions of the invention, the surface to be plated should be catalytically active and free of grease and contaminating material. Where a nonmetallic surface is to be plated, the surface area to receive the deposit should first be sensitized to render it catalytically active as by the Well known treatment with an acidic aqueous solution of stannous chloride followed by treatment with a dilute aqueous acidic solution of palladiurn chloride. Alternatively, good sensitization of nonmetallic surfaces is achieved by contact with an acidic colloidal formulation formed by the admixture of stannous chloride and a precious metal chloride, preferably palladium chloride, the stannous chloride being present in stoichiometric excess based upon the amount of precious metal chloride.

The invention will be better understood by reference to the following examples where all parts were plated using the following procedure:

(1) Cut a phenolic substrate to a size of 2" by 2",

(2) Scrub part clean using an abrasive cleaner,

(3) Rinse in cold water,

(4) Immerse in a solution of a wetting agent identified as Shipley Conditioner 1159 at room temperature for one to three minutes,

(5) Rinse in cold water,

(6) Immerse in a colloidal stannic acid-palladium catalyst (identified as Cuposit Catalyst 6F) maintained at room temperature for a period of from 1 to 5 minutes,

(7)Rinse in cold water,

(8) Immerse in a Cuposit Accelerator 19 or a mild perchloric acid solution maintained at room temperature for a period of from 3 to 10 minutes,

(9) Rinse in cold water, and

(10) Immerse in the electroless plating solution of the examples for a time suflicient to provide a deposit 0 desired thickness.

EXAMPLE 1 Cadmium sulphate-12 grams. Copper sulphatel25' p.p.m. Paraformaldehyde--l0 grams.

N,N,N,N-tetrakis (2-hydroxypropyl) ethylenediamine- 17 grams.

Triisopropanolamine-S grams.

Pentahydroxypropyl diethylenediamine-0.8 gram.

Polyacrylaoxide0.025 gram.

Mercuric acetate-20 p.p.m.

Mixed sodium and potassium hydroxidesto pH about Water-to 1 litre.

The above formulation was used at room temperature to plate a phenolic substrate. Within about to minutes, deposition of cadmium was noted. Within about two hours, a bright shiny cadmium deposit was obtained.

EXAMPLE 2 The procedure of Example 1 was repeated, but the mercuric acetate was omitted from the formulation. Within about ten minutes, some deposition was initiated, but shortly thereafter, plating stopped.

EXAMPLE 3 The procedure of Example 1 was repeated, but the copper sulphate was eliminated from the solution. No plating occurred.

EXAMPLE 4 The procedure of Example 1 was repeated, but the concentration of the copper sulphate was increased to about 625 p.p.m. The solution plated at a faster rate, and again, a bright shiny cadmium deposit was obtained.

EXAMPLES 5 TO 12 The procedure of Example 1 was repeated with the amounts of cadmium sulphate and copper sulphate varied relative to each other as set forth in the following table with the results indicated:

0. Bright, dark copper colored deposit.

EXAMPLE 13 Copper sulphate12 grams.

Cadmium sulphate-3 grams.

Paraformaldehyde-10 grams.

N,N,N',N tetrakis (2 hydroxy propyl) ethylenediaminel7 grams.

Triisopropanolamine5 grams.

Pentahydroxypropyldiethylenetriamine0.8 gram.

Sodium hydroxedto pH about 10.5.

Waterto 1 liter.

The above solution was used at room temperature to deposit electroless metal over a phenolic substrate. A cop per appearing deposit was obtained which was analyzed and found to contain 15% cadmium with the rest being copper. The significance of this example is that copper rich copper-cadmium alloys can be obtained without mercury.

EXAMPLE 14 The procedure of Example 13 was repeated, but the cadmium sulphate and copper sulphate content were each at 7.5 grams per liter. The resultant alloy deposit con tained about 24% cadmium with the remainder copper. This indicates that the practical maximum amount of cadmium obtained in the deposit when the plating solution is free of mercury is about 24 or 25% 6 EXAMPLE 1s Cadmium su1phate'12.5 grams.

Copper sulphate-425 p.p.m. Ethylenediamine tetraacetic acid20 grams. Paraformaldehyde15 grams.

Mercuric acetate-0.2 gram.

Sodium hydroxide-to pH about 10.5. Waterto 1 liter.

The above bath was used to plate a phenolic substrate at room temperature. A bright, shiny copper appearing deposit was obtained.

The electroless cadmium and cadmium-copper alloys of this invention are useful for many of the same purposes as electroless nickel and electroless copper of the prior 'art. For example, electroless cadmium is a desirable deposit for corrosion resistant protective coatings and can be substituted for electrodeposited cadmium in many military specifications. Further, by virtue of the conductivity imparted by the copper to the cadmium, the alloy deposits have use in electronics fabrication such as in the manufacture of printed circuit boards.

We claim:

1. An electroless plating solution comprising a source of cadmium ions, a source of copper ions, at least one complexing agent for said cadmium and copper ions in an amount suflicient to maintain the same in solution, formaldehyde and hydroxyl radicals in an amount suflicient to provide required solution pH, the molar ratio of said cadmium ions to said copper ions being at least 1 to 50 and the total concentration of said cadmium ions and said copper ions being at least 0.02 mole per liter of solution.

2. The solution of claim 1 where the quantity of copper ions in solution exceeds the quantity of cadmium ions in solution.

3. The solution of claim 1 also containing a mercury compound in an amount sufficient to maintain the deposition of cadmium from solution, said mercury compound having an anion non-interfering with said electroless solution.

4. The solution of claim 3 where the mercury compound is present in an amount of at least 1 part per million parts of solution.

5. The solution of claim 3 where the mercury compound is present in an amount of from 10 to parts per million parts of solution.

6. The solution of claim 4 where the cadmium to copper molar ratio ranges between about 1:50 and 1000:1.

7. The solution of claim 4 where the cadmium to copper molar ratio ranges between about 1:1 and 500:1.

8. The solution of claim 7 where the total of the cadmium and copper in solution ranges between about 0.02 and 2. 0 moles per liter of solution.

9. The solution of claim 4 where the copper is contained in solution in only trace quantities.

10. The solution of claim 4 where the copper is contained in solution in an amount of at least 5 parts per million parts of solution.

11. The solution of claim 4 where the copper is contained in solution in an amount of from about 100 to 250 parts per million parts of solution.

12. The solution of claim 4 where the amount of copper in solution exceeds the amount of cadmium.

13. The solution of claim 4 Where the amount of cadmium in solution exceeds the amount of copper in solution.

14. The solution of claim 4 where the complexing agent for the solution is a complexing agent both for copper and cadmium.

15. The solution of claim 14 where the complexing agent is selected from the group of polyamines and alkanolamincs.

16. The solution of claim 4 having pH of at least 10.0.

17. The solution of claim 4 containing a polymer dispersible in solution in an amount of at least 10 parts per million parts of solution said polymer being selected from the group of lower alkyl cellulose ether, hydroxy lower alkyl cellulose ether, hydroxy lower alkyl starch, polyvinyl pyrrolidone, polyvinyl alcohol, gelatine, a peptone, polyacrylamide and mixtures thereof.

18. An alkaline electroless cadmium plating solution comprising a predominant source of cadmium ions, a source of copper ions in minor amounts sufiicient to initiate deposition of cadmium on a catalytic surface, the total of said cadmium ions and copper ions exceeding 0.02 mole per liter of solution, a mercury compound soluble in solution in an amount sufficient to sustain the deposition of cadmium from solution, said mercury compound having an anion non-interfering with said electroless cadmium solution; formaldehyde as a reducing agent for said cadmium ions, at least one complexing agent for said metal ions in an amount sufiicient to maintain the same in solutionand free hydroxide to provide required solution pH.

' 19. The solution of claim 18 where the mercury compound is in solution in an amount of at least 1 part per million parts of solution.

20. The solution of claim 18 where the mercury compound is in solution in an amount of from to 100 parts per million parts of solution.

21. The solution of claim 19 where the copper ions are present in solution in an amount of at least 5 parts per million parts of solution.

, 22. The solution of claim 19 where the copper ions are present in an amount of from about 100 to 250 parts per million parts of solution.

23. The solution of claim 19 where the cadmium is in solution in an amount of from 0.02 to 2.0 moles per liter of solution.

24. The solution of claim 19 having a pH of at least 10.0.

25. The solution of claim 19 where the complexing agent is selected from the group consisting of polyamines and alkanolamines.

26. The solution of claim 19 also containing a polymer dispersible in alkaline solution, said polymer being selected from the group of lower alkyl cellulose ethers, hydroxy lower alkyl cellulose ethers, hydroxy lower alkyl starch, polyvinyl alcohol, polyvinyl pyrrolidone, gelatine,

a'peptone, polyacrylam'ide and mixtures thereof.

.27. The solution of claim 26 where the polymer is selected from the group oflower alkyl cellulose ethers,

hydroxy lower alkyl cellulose ethers, hydroxy lower alkyl starch, polyvinyl alcohol, polyvinyl pyrrolidone gelatine,

a peptone, polyacrylamide and mixtures thereof.

28. The solution of claim 26 where the polymer has members selected from the group of secondary and teras 'a'reducingagent for said metal ions, a rnercurycompound having an anion non-interfering with said electroless solution in an amount sufiicient to maintain deposition of cadmium, and hydroxide ions to provide required solution pH; said copper and cadmium ions being in solu tion in relative amounts such that the alloy deposit obtained from said solution contains at least one-half percent .by weight of each of cadmium and copper and the total of said cadmium ions and copper ions being in excess of 0.02 mole per liter of solution.

30. The solution of claim 29 where the amount of cadmium in solution exceeds the amount of copper.

31. The solution of claim 29 where the amount of copper in solution exceeds the amount of cadmium.

32. The solution of claim 29 where the cadmium to copper weight ratio varies from 15:1 to 1:15.

33. The solution of claim 29 where the combined amount of cadmium and copper in solution varies from 0.02 to 2.0 moles per liter of solution.

34. The solution of claim 29 where the same complexing agent is used for both copper and cadmium.

.35. The solution of claim 34 where the complexing agent is selected from a group of polyamides and alkanolamines.

36. The solution of claim 29 Where the mercury compound is in solution in an amount of at least one part per million parts of solution.

37. The solution of claim 29 where the mercury compound is in solution in an amount of from 10 to parts permillion parts of solution.

38. The solution of claim 29 where the solution also contains a polymer dispersible in solution, said polymer being selected from the group consisting of lower alkyl cellulose ethers, hydroxy lower alkyl cellulose ethers, hydroxy lower alkyl starch, polyvinyl alcohol, polyviny pyrroidone, gelatine, a peptone, polyacrylar'nide and mixtures thereof.

39. The solution'of claim 38"where the polymer is selected from the group consisting of lower alkyl cellulose ethers, hydroxy lower alkyl cellulose ethers, hydroxy lower "alkyl starch, polyvinyl alcohol, polyvinyl pyrrolidones, gelatine, a peptone, polyacrylamide and mixtures thereof.

40. The solution of claim 38 where the polymer has a member selected from the group of secondary and tertiary nitrogen atoms and hydroxyl groups.

. .R i se C e! UNITED STATES PATENTS 3,370,974 2/1968 Hepfer 106-1 3,403,035 9/ 1968 Schneble et al. '106l LORENZO B. HAYES, Primary Examiner US. 01. X.R.

l1747 A, 13 0 E, R 

